240.4 (D)

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mbrooke said:
What is the purpose of this rule?
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Not sure what you want to know. This section gives the overcurrent protective device that is allowed for these conductors. If you look in the table at the 75C column for these conductors you would think that you could use a larger breaker than what this section requires. I guess the NEC does not want you to use 12 awg with a 25 amp breaker except in certain situations so they state it here that you cannot use 12 awg any higher than 20 amps.

There are exceptions of course but this sections is limiting the overcurrent protective device for those conductor sizes
 
At one time many of the devices that commonly are connected by #14-#10 wire were 60? rated.
In one class I attended we were told that 240.4(D) served as a back-up to 110.14.
How factual that is I don't know.
The smaller conductors were added in '08. Prior to that I am unsure where the OCP rules for #18 and #16 were/
 
Dennis Alwon said:
Not sure what you want to know. This section gives the overcurrent protective device that is allowed for these conductors. If you look in the table at the 75C column for these conductors you would think that you could use a larger breaker than what this section requires. I guess the NEC does not want you to use 12 awg with a 25 amp breaker except in certain situations so they state it here that you cannot use 12 awg any higher than 20 amps.

There are exceptions of course but this sections is limiting the overcurrent protective device for those conductor sizes
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I guess why it exists. It just puzzles me since we are allowed to use the 75*C column for larger wires but restricted to special applications in smaller ones.






augie47 said:
At one time many of the devices that commonly are connected by #14-#10 wire were 60? rated.
In one class I attended we were told that 240.4(D) served as a back-up to 110.14.
How factual that is I don't know.
The smaller conductors were added in '08. Prior to that I am unsure where the OCP rules for #18 and #16 were/
Click to expand...


That makes more sense, but wouldn't 110.14 in itself suffice? Since almost everything is now 75*C, Id imagine the rule could be lifted.
 
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